Numerous applications exist in which it is desired to form repeating patterns having a small pitch (for example, a pitch of less than about 50 nanometers). For instance, integrated circuitry fabrication may involve formation of repeating patterns of memory storage units (i.e., NAND unit cells, dynamic random access [DRAM] unit cells, cross-point memory unit cells, etc.). Additionally, nanoscale mechanical, chemical, biological and other electrical devices and systems are being fabricated.
Photolithography is a conventional method used for fabrication of nanoscale devices and systems. Photolithography uses incident radiation of a selected wavelength to pattern a photosensitive material. The exposed or un-exposed portions of the photosensitive material are then selectively removed relative to the other. The material which remains is used as a mask in patterning underlying substrate material which is exposed through openings in the mask.
A continuing goal in integrated circuitry fabrication is to increase circuit density, and accordingly to decrease size of individual integrated circuit components. Thus, there is a continuing goal to form patterned masks to have increasing densities of individual features and less space between adjacent features. If photolithography alone is used to pattern integrated circuit components, circuit density is limited by a threshold dictated by the minimal attainable feature size using the particular photolithographic technology. The minimum feature size is dictated by, for example, a wavelength utilized during patterning of the photosensitive material. Conventional photolithographic processing methods are not readily capable of accommodating fabrication of structures and features much below the 100 nanometer level.
Methods have been developed which can be used in combination with photolithography or other processing to push the minimum attainable feature size to smaller dimensions than may be achieved with photolithography alone. One such method is a procedure comprising use of a block copolymer material to form a pattern between a pair of photolithographically-patterned, or other patterned, features. Block copolymer materials spontaneously assemble into periodic structures by microphase separation of the constituent polymer blocks upon annealing at a suitably high temperature. Such form ordered domains at nanometer-scale dimensions between the photolithographically-patterned, or other patterned, features. Following such self-assembly, one block of the copolymer can be selectively removed thereby leaving a mask having nano-sized features and openings through which underlying substrate material can be processed.
Copolymers are polymers derived from two or more monomeric species. Block copolymers contain two or more homopolymer subunits linked by covalent bonds. Two example block copolymer materials are polystyrene-b-poly2-vinylpyridine and polystyrene-b-poly4-vinylpyridine, and which are referred to herein as PS-b-PXVP where “X” is 2 or 4.
Conventional processes by which PS-b-PXVP is manufactured result in impurities being received in the composition. Example impurities include lithium salts, for example lithium chloride. Some commercially available PS-b-PXVP has lithium salt on the order of 400 to 500 parts per million by weight in the composition. Such can result in lithium salt particles on the substrate on which the mask is being formed, and about which block copolymer patterns can form upon anneal as opposed to solely relative to the previously patterned features.